Any action or proceeding in which the validity of the exclusion of territory from the improvement district or of any of the proceedings in relation thereto is contested, questioned, or denied shall be commenced within three months from the effective date of the resolution of exclusion; otherwise, the exclusion and all proceedings in relation thereto shall be held to be valid and in every respect legal and incontestable.
Source:
Section 72089.5, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=WAT§ionNum=72089.5..